I. Field
The present disclosure relates generally to communication, and more specifically to techniques for performing time tracking at a receiver in a communication system.
II. Background
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation technique that can provide good performance for some wireless environments. OFDM partitions the overall system bandwidth into multiple (K) orthogonal frequency subbands, which are also called subcarriers, tones, bins, and so on. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data. Up to K modulation symbols may be sent on the K subbands in each OFDM symbol period.
In an OFDM system, a transmitter typically transforms the modulation symbols for each OFDM symbol period to the time domain with a K-point inverse fast Fourier transform (IFFT) or inverse discrete Fourier transform (IDFT) to obtain K time-domain chips. To combat delay spread in a communication channel, the transmitter repeats some of the K chips to form an OFDM symbol. The repeated portion is commonly called a guard interval or a cyclic prefix. The guard interval is used to combat intersymbol interference (ISI) and inter-carrier interference (ICI) caused by delay spread, which is the time difference between the earliest and latest arriving signal paths at a receiver.
The receiver performs the complementary processing and removes the guard interval in each received OFDM symbol. The receiver then transforms K time-domain samples for each received OFDM symbol to the frequency domain with a K-point fast Fourier transform (FFT) or discrete Fourier transform (DFT) to obtain K received symbols for the K subbands. The receiver then performs detection on the received symbols to recover the transmitted modulation symbols. The receiver typically maintains a time tracking loop that determines the proper placement of an FFT window for each received OFDM symbol. This FFT window indicates which samples should be retained and which samples should be discarded. Data detection performance is greatly affected by the placement of the FFT window.
There is therefore a need in the art for techniques to perform time tracking at a receiver to achieve accurate placement of the FFT window.